A data set of quadruple-label experiments in the human forearm (with insulin clamped at different levels) and myocardium (in different heart working conditions) is available for modeling analysis. An extravascular, extracellular, permeant nonmetabolizable and permeant metabolizable glucose tracers were used. The aim of these studies is to assess the extent to which glucose transport and metabolism in vivo are influenced in the forearm by different levels of insulin and in the myocardium by different heart beating rates. Experimental results will be analyzed using the compartmental and the distributed models approach, and a comparison will be attempted between the results obtained with each method. While analyzing these complex systems, new modeling aspects (models of flow heterogeneity in humans in vivo, better estimation of precision of model parameters) will be explored. Specific Aims: 1. To develop a mathematical model of capillary-tissue exchange suitable for analysis of the experimental results; 2. To determine the transport and metabolism of glucose in human muscle tissue, and its control by insulin, using the multiple indicator dilution technique.